Traffic paints are classified by "auto-no-track-time", defined as the period of time after which a passenger car can pass over a freshly applied marking without tracking, i.e., without pick-up of material by a tire and redeposition of the material on the road surface with subsequent revolutions. Paints with a 90 second or less auto-no-track-time are preferred because they do not require placing cone barriers on the road to prevent motorists from driving over the wet paint. The placing and retrieval of the cones is costly and hazardous to the personnel handling the cones.
Paints that do not require coning are usually solvent-based, comprised of alkyd or chlorinated rubber vehicles. Under severe service conditions, however, such materials may have a useful life of six months or less. More durable marking materials generally are solventless systems and are based on epoxy or polyester resins, or hydrocarbon or rosin ester thermoplastic materials. Their service life when properly applied can approach three or more years.
However, polyester and epoxy liquid compositions have auto-no-track-times of 8-10 minutes and 10-15 minutes, respectively, and therefore require coning. Alternatively, a heavy application of glass beads (20-25 pounds versus a normal 6-8 pounds per gallon of material applied) can be used with epoxy resins to eliminate coning, but that significantly increases the cost. Thermoplastic hydrocarbon or rosin ester coatings dry quickly by cooling, but become soft in hot weather and require expensive application equipment.
Urethane coatings have long been used for exterior applications where high performance is required, e.g., for bridge paints and aircraft finishes. They have not been used commercially for durable traffic paints, however, because the freshly applied coating would remain wet too long due to slow solvent release.
It would be desirable to make available a traffic paint composition that has the durability provided by a nitrocellulose-urethane vehicle, while having a drying time short enough to permit its use without coning.